Lower fuel consumption of automobiles is recently strongly required due to increased oil prices and severe environmental problems. With respect to piston rings, the reduction of ring tension, and the use of hard amorphous carbon coatings, which are expected to have low friction, etc. have been investigated. Though they actually contribute to the reduction of fuel consumption, the reduction of ring tension is reaching the limit from the aspect of sealability, and hard amorphous carbon coatings are easily peeled from the substrate because of poor adhesion due to high compression stress during the formation of coatings.
As a technology for improving the adhesion of a hard amorphous carbon coating to a substrate, various methods such as a method of relaxing the internal stress of a hard amorphous carbon coating per se, a method of forming an intermediate layer having medium properties between a substrate and a hard amorphous carbon coating, etc., have been proposed.
JP 11-172413 A discloses a method for relaxing internal stress in a hard amorphous carbon coating per se, by dispersing carbides of one or more elements selected from the group consisting of Si, Ti, W, Cr, Mo, Nb and V in the coating. This coating has not only improved adhesion due to the relaxed internal stress, but also excellent initial conformability because of relative softness, which is unexpected feature as a hard amorphous carbon coating.
Conventional intermediate layers are as thick as 50 nm or more, and when an amorphous carbon coating is formed on such a thick intermediate layer, sufficient adhesion is not obtained to machine parts, etc. used under extremely high contact pressure. In view of this problem, JP 2001-316800 A discloses an intermediate layer made of at least one selected from the group consisting of elements in Groups IVa, Va, Via and IIIb, and Group IVb (excluding C), or carbide thereof, and having thickness of 0.5 nm or more and less than 10 nm for remarkably high adhesion.
JP 2006-250348 A discloses an intermediate layer having excellent adhesion to both of a substrate and a DLC coating, which comprises a metal capable of reducing oxides on the substrate surface (for example, a metal such as Si, Ti, Cr, etc., whose oxide has a standard formation free energy of −600 kJ or less at 100° C.), thereby having an oxygen concentration increasing from the surface side to the inside; a layer of carbide or amorphous carbon having high affinity to the DLC coating being formed on the intermediate layer, such that the concentration of carbon decreases from the surface side to the inside.
JP 2007-169698 A discloses the formation of a low-density, hard amorphous carbon coating as an intermediate layer between a substrate and a high-density coating having a high sp3 bond (diamond bond) ratio, in place of an intermediate layer of metal, etc. This low-density, hard amorphous carbon coating looking bright in a transmission electron photomicrograph decreases residual stress and relaxes internal stress, thereby improving adhesion. It also describes that continuously changing density is more preferable.
JP 2008-25728 A discloses the formation of a primary layer made of at least one of Cr, W, Ti and Si, and an intermediate layer made of the above metal and carbon, a ratio of carbon increasing from the primary layer side to the surface side, under a hard amorphous carbon coating, instead of forming only an intermediate layer.
As a multi-layer intermediate layer, JP 2013-91853 A discloses an intermediate layer having a laminate structure comprising a first Cr layer, a CrN layer and a second Cr layer in this order from the base material (substrate) side.
However, the above intermediate layers are still insufficient, failing to overcome such problems as cracking, chipping and peeling of hard amorphous carbon coatings.